Question-------------------------
Followup To
Question -
I have a low spongy brake pedal. I have replaced both wheel cylinders and the master cylinder. My brother and me bled the entire system the other day to my satisfaction.
Then when I started the engine the pedal went to the floor. I can pump up the brakes and they stay hard when the engine is not running, but when the engine is running, the pedal goes soft and spongy.
Answer -
Is this brake system the NORMAL brake system or is it a HYDRAULIC ASSIST system ?. most likely it is a normal brake system so see below.
IF your POSITIVE that you have ALL the air bled out of the entire brake system INCLUDING bleeding the FRONT brakes as well as the rear and did you follow the directives to the LETTER that came with the master cylinder, Then thre only other possibility is that it would appear that If you listen carefully at the brake pedal when you apply the brake pedal can you hear an air leak ?.
If so, You may have a split in one of the two brake booster diaphrams of which requires removal, disassembly and replacement of the big rubber diaphram OR replacement of the booster.
I ALWAYS mount my master cylinder in an adapter I made for my wor bench so I can fill the master cylinder and hand blled all the air out, Plug off the brake line outlets and apply 150 lbs of pressure to the plunger in the master cylinder and make sure it holds pressure for 15 minutes. I have had many faulty NEW or REBUILT master cylinders, So do NOT overlook this possibility.
My thoughts SAY brake booster problem per above.
autohelp
I appreciate the quick reply, I am fairly confident we bled all the air out, but am not absolutely certain. I did bench bleed the master cylinder.
Here is some more info that I now remember, hopefully it will help. When the engine is running, you can pump up the pedal and it will stay up, as long as you keep pressure on the pedal, but if you release pressure off the pedal for any length of time, the next time you press the pedal it will go to the floor, then you can pump it up again.
If the engine is not running everything feels good.
When I listen at the brake pedal I am assuming I will need the car to be running, is this correct?
Thanks again,
Doug
AnswerThat is correct engine running. You will hear a rush of air coming from the backside of the vacuum booster into the brake pedal arm at the firewall upon brake application and in some cases depending upon the size of the air leak you can still hear and feel the air while holding the brake pedal down.
Here is another test for a faulty, Although new or rebiult master cylinder.
Engine OFF during complete test below.
1. Pump UP the brakes to full pedal.
3. release the brake pedal.
3. wait 10 seconds.
4. Apply the brake pedal.
5. If the pedal goes down more than 3 inches lower than in step #1, Replace the master cylinder.
Master cylinders that set on the parts store shelf for any length of time and or are not rebuilt correctly can cause the inner CUPS (seals) on the main shaft to get hard and allow fluid to leak past the cups.
Had you TESTED it as I said prior you would have felt that you had more travel on the cylinder using the hand application method and the pressure gages would have shown reduced pressure application.
QUESTION: Are we dealing with FRONT DISC and REAR DRUM brakes or ALL DISC BRAKES ?.
When you said WHEEL CYLINDER, I immediately thought FRONT DISC and REAR DRUM. It makes a difference especially if the REARS are DISC as some models have what looks like a large CROSS cut acroos the surface of the application piston of which is for the automatic parking brake adjustment.
If you lightly apply the brake pedal JUST to stop one whell from turning, Do ALL the wheels also prevented from turning ?.
1. If your POSITIVE you have ALL the air out from ALL the wheels and lines.
2. If your POSITIVE you have the rear brake shoes (if drum brakes) adjusted correctly to the drums.
3. If you hear no significant air leak at the brake booster.
Replace the master cylinder. I have seen my share of faulty ones.
To be hONEST, Every diagnosis is in the actual FEEL of the brake pedal and many years of working on brakes. Just the FEEL of the pedal tells the technician what the problem is and since there is no way for me to do this, Trying to diagnose your problem this way is very difficult.
Let me know.
autohelp
